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1. Mythic Changes

Besides all adds and the boss having more health and all abilities doing
more damage, there are a couple of new abilities and mechanics in the Mythic
mode of the fight.

1.1. Phase One

Adds behave exactly in the same way as in Normal and Heroic mode, although spawn
patterns may be different. The explosion of Nightmare Ichors is slightly different
though. Instead of only dealing damage to nearby players, it also applies a stacking DoT
to all raid members, ticking twice in 4 seconds for a mild amount of Shadow damage.

There is also an entirely new mechanics called Death Blossom, which seems to be
cast approximately once per minute by the Eye of Il'gynoth. It has a 15-second
cast time. When the cast begins, 15 Death Blossoms (small void zones)
start appearing at random locations in the room. When the cast finishes, the blossoms
detonates, dealing high Shadow damage to all players standing in the blossom and
granting them 50% increase movement speed for 6 seconds. If there is no one standing
in the blossom, then the entire raid takes a mild amount of Shadow damage and then
low Shadow damage every 2 seconds for 50 seconds.

1.2. Phase Two

In Phase Two, there are two new mechanics.

Violent Bloodburst is cast twice per Phase Two. It creates damaging
patches of blood that expand outwards from the boss, eventually leaving only two safe
zones in the room on each side of the boss, in the form of a small pizza slice.

Shriveled Eyestalks are small tentacles that are only present in the
room the second time you enter Phase Two. They have low health and keep trying to
cast Mind Flay, which can be interrupted.

2. Strategy

As we are going to see, the strategy for Mythic is very similar to Heroic mode.

2.1. Phase One

In Phase One, the main difference is the way Nightmare Ichors need
to be killed. Because of the stacking debuff Nightmare Explosion
applies to all raid members in Mythic, Nightmare Ichors need to be killed one
or two at a time, every 4 seconds at least, throughout the fight.

When killing 2 Nightmare Ichors at the same time, they will usually not die
right at the time same, but it is important that the second one dies less than
2 seconds after the first one. Otherwise, the debuff has the time to tick once
before being refreshed and dealing 2 ticks of damage at 2 stacks (so you want to avoid
this extra tick of damage at one stack).

To prevent Nightmare Ichors from blowing up in the wrong place, make sure to have
players spec into knockback effects, such as Typhoon. Death Grip from
Death Knights also work.

When Deathglare Tentacles and Corruptor Tentacles spawn at
the same time (it can happen several times per fight). This requires good raid
coordination to interrupt the Deathglare Tentacles and kill the Corruptor
Tentacles as soon as possible. For Corruptor Tentacles that spawn on
the edges of the room, there is no need for the entire raid to focus them down.
Instead, simply send classes that excel at killing them or players who have
available cooldowns. This way, you can leave a good part of the raid to focus
on interrupting Mind Flay. Failing to interrupt Mind Flay will
very quickly result in deaths in Mythic mode.

Death Blossom only adds a positioning constraint, because you need
a player in each of the void zones it creates. The void zone seem to spawn
in a circle-like pattern in the center of the room. The way Serenity handles it
is by assigning each group to a quadrant of the circle. For example: Group 1 takes
care of the top left void zones, Group 2 of the top right void zones, Group 3
of the bottom left void zones, and Group 4 of the bottom right void zones. Players
fixated by Nightmare Ichors should avoid soaking, instead continuing to kiting so
that the Nightmare Ichor does in front of the Eye of Il'gynoth.

2.2. Phase Two

In Mythic mode, you need to contend with Violent Bloodburst. Once the
patches of blood start spawning, the location of the safe zones is easy to guess.
It is important to reach one of them as soon as possible, as getting hit
by a patch not only deals damage, but it also pacifies you, making you temporarily
unable to attack.

Right as Violent Bloodburst is about to end, random players are debuffed
with Cursed Blood. There is no need to panic. Players need to wait a bit
for the patches of blood to despawn before positioning themselves for Cursed Blood.

The second Violent Bloodburst+Cursed Blood combo should be
handled in the same way, except that players who are debuffed with Cursed Blood
need to run out of the room to detonate. There is no time to spread in the
room, because Dark Reconstitution would end before detonations happen.
Players who are not debuffed can stay in the room for a few more seconds to
deal some extra damage, before the cast of Dark Reconstitution finishes. It is
important that the debuffed players leave a safe spot right at the exit of the
room where non-debuffed players can stand for a bit while waiting for the
detonations.

The second time you enter the room, you will first have to kill the 5
Shriveled Eyestalk tentacles. They spawn in a star pattern around the
boss after a few seconds. Killing and interrupting them is the initial
priority. Once they are dead, Il'gynoth is once again the priority. When you
get the second Violent Bloodburst+Cursed Blood combo, there is
no need for debuffed players to move, because the raid would die before the
detonations occur, if Dark Reconstitution is allowed to finish
casting.